In recent years, acoustic reproducing apparatuses using an open-air-type head phone have been found in large number among acoustic reproducing apparatuses such as a small-sized stereo cassette tape player and portable CD player due to the more increased outdoor application in the acoustic field.
The above-mentioned open-air-type head phone includes what is generally called a head phone with a head band which covers the ears and an inner-type head phone loaded in the ears; but the above-mentioned acoustic reproducing apparatuses using these head phones include cassette tape players, portable CD players, radio receivers with FM stereo and apparatuses combining these apparatuses, any of which is of a small size which is convenient for carrying. Then, even in the case of outdoor listening, owing to the open-air type as mentioned above, the sound and alarm of vehicles, signal sound and the like can be listened to, and sound reproduction and listening can be performed without any danger while walking outdoors.
However, the configuration as mentioned above is an open-air-type and allows the outside voice to be heard, therefore having a problem of being affected directly by the surrounding noise and leaking the reproduced sound from the head phone into the surroundings.
This problem means that, as shown in FIG. 10, measurements of noise frequency characteristics in an environment of listening in an electric car and in a motor car result in a noise frequency characteristic in electric car B and a noise frequency characteristic in motorcar C in contrast with a flat frequency characteristic A of the acoustic reproducing apparatus. Both the frequency characteristic B and C have high noise levels of low frequency band and low noise levels of high frequency band although the frequency bands thereof somewhat deviate from each other. Accordingly, there exists a problem that where the output of the reproducing apparatus having the frequency characteristic A is intended to be listened to, the sound of the low frequency band is buried in noise and is hard to be listened to either in an electric car or in a motorcar, and the voice of the high frequency band is emphasized, and that when the sound volume is increased to listen better to the low frequency band, the high frequency band is more emphasized. This causes a similar problem also, for example, in the case of listening alone through the head phone in an electric car or listening alone through the head phone in a motorcar.
Also, in an electric car, the open-air-type head phone causes a problem of sound leakage to the surroundings. FIG. 11 is a graph showing a sound leakage frequency characteristic D of a conventional open-air-type head phone measured under a condition as shown in FIG. 12. This means that measurements of sound leakage are made by a sound level meter M positioned apart by l (about 30 cm) from an ear La in the state that a head phone H is attached to the ear La of a listener L.
Here. the surrounding environment is a soundproof chamber, and the sound level meter M uses a A-curve filter. As a result, as shown in FIG. 11, the sound leakage increases at frequencies above 1 kHz, thereby showing a sound leakage frequency characteristic Da and showing such a frequency characteristic Db that the dark noise level of the soundproof chamber is flat in the frequency band below 1 kHz. Thus, it is found that the sound leakage is generated largely in the high frequency band, and this becomes a problem as a cause of sound leakage in the case of listening through the above-mentioned open-air-type head phone.
As described above, conventionally, when reproduced sound is intended to be listened to in an electric car or in a motorcar, low-pitched tones are very hard to be listened due to the noise In that place, and by resultant increasing of the sound volume, high-pitched tones are extraordinally emphasized, and a problem is caused that the listener himself gets tired of listening. Furthermore, as is the case with listening in an electric car, where many people are present in the surroundings, the noise from the surrounding electric car also exists, and the sound volume is likely to be made large; consequently the surrounding people hear the voice leaking from the head phone as annoying noise, and causes a problem of annoyance.